try this

.github/workflows/external-pr-label.yml

@@ -0,0 +1,25 @@
+name: External PR Label
+
+on:
+  pull_request:
+
+jobs:
+  external-pr-label:
+    # if: ${{ github.event.pull_request.head.repo.fork }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      pull-requests: write
+      issues: write
+    steps:
+      - name: Add External label
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: | # js
+            await github.rest.issues.addLabels({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.issue.number,
+              labels: ['External']
+            });